This invention generally relates to microwave ovens and more specifically to microwave ovens for use in vehicles such as cars, trucks, aircraft, and boats.
Microwave ovens designed for portable operation and operation in vehicles have been proposed previously. U.S. Pat. No. 4,667,075 of Sakurai proposes using a three-phase AC alternator to supply power. U.S. Pat. No. 5,276,300 of Low et al. describes a microwave oven containing essentially an inverter to allow 12 volt DC operation. AGOR technologies of Verson, France manufactures a similar product. These devices, operated at power levels of about 0.6-1.0 kW nominal, require DC input currents from a 12 volt source of approximately 100 amperes. This much current requires direct connection to the vehicle battery with very heavy gauge wire or the dedicated alternator of Sakurai and implies mechanical and electrical installation skills beyond the range of the average consumer. The oven must also be somehow mounted to the vehicle so that it does not move around. The microwave ovens previously described are mechanically very similar. They feature a cavity with a front opening door, that is, with a vertical hinge. The timer and control keypad is mounted vertically in proximity to the door and is very similar to or modifications of conventional consumer microwave ovens.
Conventional consumer microwave ovens have power ratings in the 500 W-1000 W range. These ratings represent the power delivered into the oven cavity. The nameplate power consumption of these microwave ovens is typically 50-75% higher. For example a Magic Chef MC-03JS Compact Microwave oven is rated at 500 Watts and is rated at a power consumption of 750 Watts. While these ovens could be powered by a DC to AC inverter of appropriate output, the inverter would have to be hardwired with heavy gauge wire, such as No. 0, to the vehicle battery or alternator in order to receive the necessary current. If we assume a 90% inverter efficiency the 500 Watts oven mentioned above would cause a typical inverter to draw a current of about 60 amperes at a nominal 13.8 volts.
This invention provides a small, low-power, magnetron-type microwave oven. The power is often less than 300 Watts, typically in the vicinity of 200 Watts. The microwave oven described herein is designed to be powered by a vehicle""s xe2x80x9caccessory powerxe2x80x9d or xe2x80x9ccigarette lighterxe2x80x9d jack and to be held in place on a seat by a standard vehicle seat-belt. Also described is an upward-opening door and plastic radio-transparent microwave cavity liner designed to contain spilled foods and facilitate cleaning. Methods of retaining food containers which hold the foods being heated, as well as other safety and convenience features useful for real-world operation in the mobile environment, are described.
It is envisioned that the device described will find wide application in cars, trucks, recreational vehicles, aircraft, boats and simple living situations which do not provide conventional AC mains and in which consumers are unlikely to perform, or have performed, involved installations. It can be used for many tasks, from heating baby bottles and meals in properly designed containers, to heating the morning""s coffee, to warming automotive gasket materials in cold climates during repair operations.
In one embodiment, a high frequency switching power supply generates the required filament and high voltage necessary for magnetron operation. This is more efficient than suffering DC/AC inverter losses in addition to the losses seen in typical prior art microwave oven power transformers. The magnetron is the type whose magnetic field is generated by permanent magnets.
The only readily available electrical outlet or jack in most modern cars and trucks and smaller boats is the familiar xe2x80x9ccigarette lighterxe2x80x9d or xe2x80x9caccessory powerxe2x80x9d socket. This socket is familiar to millions of users who power mobile phones, radar detectors, small inverters and other convenience devices from them. These jacks are typically fused at 15-30 amperes. This typically limits total electrical power input to the microwave oven described here to a range of approximately 200-400 Watts. This corresponds to a typical microwave power rating of about 125-250 Watts. Most users of microwave ovens are used to larger power inputs as mentioned previously. Therefore the cooking time in this low power microwave oven will be longer than the xe2x80x9cstandardxe2x80x9d times experienced in larger, more powerful units. For example, if it takes 1 minute to heat a cup of soup to serving temperature in a 750 Watt microwave oven found in a typical American kitchen, it will take three minutes nominally in a 250 Watt unit. In fact, given the significant increase in cooking times, there will be thermal losses including radiational and convective losses which will tend to cool the food being heated and which will lengthen the required time beyond the simple ratio of power. In one mode, the microwave oven described here takes the xe2x80x9cstandardxe2x80x9d cooking time for a 750-900 Watt microwave oven and automatically multiplies it by an appropriate factor to arrive at the correct heating time for this lower-power oven. In another mode, the user enters a desired heating interval which is not multiplied by any factor.
The present microwave oven also preferably senses input current and voltage. It shuts down if excessive current is drawn to protect the xe2x80x9caccessory socketxe2x80x9d fuse or if voltage drops to a level just above that which corresponds to the vehicle battery being unable to start the vehicle.
A microwave oven, even a small one, is a substantial mass, and may contain foods which are near or at the boiling point. It must be securely mounted within the vehicle cockpit, such that in case of an emergency maneuver, sudden acceleration, or crash, it does not fly about the cockpit causing injury or burn hazards to passengers or damaging other components of the vehicle. The present microwave oven typically has split loops affixed to the outside of the cabinet which facilitate securing the microwave oven in a seat with the seatbelt much as a child""s safety seat would. In the event of a collision the microwave oven described here shuts down. It also has the usual safety features such as door interlocks and over-temperature sensors found in virtually all consumer microwave ovens. Conventional mounting points are provided for situations in which the device may be bolted down.
The intake and exhaust for the cooling air required by the magnetron are also preferably placed on the top surface such that the airflow is not impeded as it would be if mounted on a seat with downward or side facing intakes and exhausts. The keypad is also mounted on the top surface for easy access.
The microwave oven described herein also normally has an upward opening door allowing access to the oven cavity. The cavity preferably has a radio-transparent, removable, easily cleaned, plastic liner made of heat resistant plastic which contains any spills which may occur. These two features help insure that, even if food somehow should spill within the oven, it will not leak out into the vehicle cabin. The door is preferably retained shut by a locking mechanism when the oven is in operation and may stay in that state until an unlock button is pushed on the unit""s keypad.
Since the vehicle may be in motion while the microwave oven of the invention is in use, means are typically provided of fixing the food container in place so that it does not xe2x80x9cbounce aroundxe2x80x9d and spill its contents or damage the cavity walls. In one embodiment, a detented stud is centrally affixed to the center of the carousel. There are mating cavities molded into xe2x80x9cmicrowave safexe2x80x9d plastic heating containers with locking lids and appropriate vents. The food is placed into the container, the vented lid is affixed with a latching lip or similar arrangement, and the cavity, typically in the center of the container is pressed down onto the detented stud which retains it in place. Food portions in disposable heating containers, such as xe2x80x9cChef Boyardee(copyright) Lasagna, Microwave Meals, (10.5 oz. size)xe2x80x9d can be heated with the appropriate mating cavity molded in.
Alternatively an inverted pyramidal well is molded into a radio-transparent, heat resistant plastic support sheet. Food in appropriate containers is placed down into the well until contact is made with the walls of the well. Alternatively a curved non-metallic spring-loaded radio-transparent wall could be used as retention device.
A carrying handle is typically molded into the top surface to facilitate handling of the mobile microwave oven. A DC input cable terminated in a xe2x80x9ccigarette lighterxe2x80x9d plug or other suitable connector of sufficient length and sufficient gauge is also typically provided so that the microwave oven can be installed in the back seat of long vehicles while being plugged into the xe2x80x9ccigarette lighterxe2x80x9d or xe2x80x9caccessory socketxe2x80x9d which is usually proximal to the vehicle""s front seat.